Found this FAQ from googling Avis First changes:
http://www.avis.com/car-rental/conte...t-US_en-048467
Not a great deal more detail than has already been discussed but it does confirm all Avis First migrates to Avis Preferred Plus:
Will I be automatically converted to the new Avis Preferred Plus tier so I can continue to receive free upgrades when available?
All Avis First members will be automatically converted to the new Avis Preferred Plus tier unless notified otherwise